The present invention relates to ammunition and particularly ammunition for use in conventional small arms weapons having rifled or non-rifled barrels.
Conventional bullets for a rifled barrel usually have a lead core with a surrounding copper jacket of a diameter which is nominally the same as the groove diameter and is thus slightly oversize or an interference fit with regard to the bore diameter of the barrel of the weapon with which it is intended to be used, the copper jacket of the bullet being engraved and slightly compressed during its passage down the barrel of the weapon by the helical rifling grooves in the barrel. The bullet is spun by the rifling grooves to stabilize its flight, but a considerable proportion of the energy produced by the propellant in the casing containing the bullet is lost through friction between the bullet and the rifle barrel caused by the engraving of the bullet, the friction generating heat in the barrel. Particularly with weapons that fire fully automatically, heat generated by the friction of the bullets passing through the barrel can be a serious problem, causing rapid barrel erosion and, at worst, the barrel to bulge or burst.
A conventional shotgun slug is a hollow, cylindrical lead cup with a domed end. However, a shotgun has no rifling grooves to cause the slug to spin and is accurate only up to a range of about 100 meters or even less, partly due to this lack of spin and partly due to the slug's unstreamlined shape which slows it quickly.
It is known to surround an undersized rifle projectile with a plastic cup (sabot) which is engraved and spun by the barrel rifling and which in turn transmits the spin to the projectile by virtue of a tight friction grip. This has the disadvantage that the sabot material must have a high coefficient of friction to maintain its grip on the projectile, with a correspondingly high friction loss in the barrel. A further consequence is that the combined mass of the sabot and projectile is less than that of a conventional projectile of the same size, which therefore has less impulse for the same energy. The advantage of this is less gun recoil, but the disadvantage is that an unmodified conventional cartridge gun, will not complete its automatic cycle because of the reduced impulse.
It is also known to use a plastic sabot to surround a flechette and to have the barrel rifling only engrave the sabot, which transfers the rotation to the flechette by mechanical engagement with the fins of the flechette, instead of by a friction grip, and therefore a low coefficient of friction material can be used for the sabot with a resulting low friction loss in the barrel. One consequence of using a flechette however is that the combined weight of the sabot and flechette is very light when compared to a conventional bullet of the same diameter and length so that a special automatic gun must be used to function with the reduced impulse. A further problem with all sabot launched projectiles is that since the sabot and projectile exit from the barrel at the same velocity, the energy of each is determined by their relative mass to one another. The heavier the sabot is in relation to the projectile, the greater is the percentage of lost energy, since the sabot serves no useful purpose as a projectile. In the prior art, the body diameter (shaft) of a flechette is small in comparison to the sabot diameter, with a resulting large proportion of mass and energy in the sabot, so that the flechette gets a relatively small amount of the total energy and is therefore the least efficient of the sabot type projectiles.
France Patent No. 1,124,740 shows a conventional shotgun cartridge which has a cylindrical slug with parallel grooves which contain secondary, spherical projectiles. Except for using a conventional base wad to seal the gas pressure behind the shot, this disclosure makes no use of the advantages of a sabot, but is nevertheless relevant to one aspect of the present invention.
It will readily be appreciated by those skilled in the art that the problems associated with the design of rifle and pistol rounds and shotgun cartridges, while having some features in common, are generally different in detail due to the different barrel environments and the uses to which the items are put.